The present invention relates to a cap for fastening devices which is useful for motor vehicle wheels, studded devices on wheels, butteries, roof lamps of four-wheel drive vehicles and like parts for preventing a third person from stealing such a part by loosening bolts or nuts fastening the part without permission or for attaching the studded device to the wheel.
FIG. 9 shows a cap 60 for fastening devices which is already known. With reference to FIG. 9, the cap comprises a cap body 61 generally in the form of a hollow cylinder for covering a polygonal engageable portion of a wheel attaching nut A. The top wall 61a of the cap body 61 has a through hole 62 close to a peripheral portion thereof. A pressure piece 65 for preventing the cap from slipping off has an outer slanting face 64 and is disposed inside an inner slanting face 63 internally provided on the cap. A pressure piece sliding screw 66 inserted through the hole 62 has an end portion screwed into a threaded bore in the pressure piece 65. A spring 69 is provided around the screw 66. The head 67 of the screw 66 is formed in its front surface with an engagement cavity 68 of specified shape. When rotated relative to the pressure piece 65 in the screwing-in direction, the screw 66 slidingly moves the outer slanting face 64 on the inner slanting face 63, moving the pressure piece 65 also radially inward simultaneously with the axial movement and thereby pressing the pressure piece 65 against the nut A with a great force, whereby the cap 60 is prevented from slipping off the nut A.
The conventional cap 60 has the advantage of being usable for fastening devices of any type, but since the cap body needs to accommodate therein the pressure piece 65 for preventing the cap from slipping off, the cap body has an outwardly projecting side portion corresponding to the pressure piece. In the case where the wheel has a small nut insertion hole in which only a small clearance is available around the nut A, there arises the problem that the projecting portion interferes with the edge of the wheel defining the nut insertion hole to render the cap 60 not fittable to the nut A.